It is well known that insulated home attics tend to become very hot during the summer. Attic fans which draw hot air out of the attic cool the attic by causing cooler air from below to flow upwardly into the attic space. Such increased air circulation, while helpful, is only partially effective in keeping attics cool during the summer (cooling) season.
It has recently been discovered that, during the cooling season, long-wave solar radiation passes through the roof and attic air space and directly heats insulation typically disposed in a layer on the floor of the attic. This heat flux causes the attic insulation to heat the air in the attic space above by convection. These findings contradict the traditional belief that the insulation is heated by the attic air.
Tests by the Florida Solar Energy Center (FSEC) have confirmed that attic overheating occurs largely because of absorption of long-wave radiation by attic insulation. The following publications discss this concept and propose installing a reflective barrier to prevent heating of attic insulation: Fairey, Designing and Installing Radiant Barrier Systems, Florida Solar Energy Center publication FSEC-DN-7-84 and Fairey, Radiant Energy Transfer and Radiant Barrier Systems in Buildings, Florida Solar Energy Center publication FSEC-DN-6-84 the contents of both of which are hereby incorporated by reference herein. The foregoing publications describe the theory of radiant barriers and recommend locations for the radiant heat barrier in the roof system. In new construction, such a radiant barrier can be constructed by applying foil sheets in a variety of positions to the roof structure above the insulation layer so as to prevent long-wave radiation from reaching the thermal insulation layer. Foil sheets are typically used, and the foil sheets must be perforated in order to prevent them from forming a vapor barrier. Such foil sheetng is effective to eliminate 95% of the radiant heat transfer which would ordinarily occur across the attic.
Energy savings resulting from the installation of a radiant barrier layer will vary from structure to structure depending on the type and quantity of insulation. However, even as to new construction, placement of foil sheets as proposed by the FSEC is highly labor-intensive. In particular, in existinq buildings, it may be in some cases extremely difficult or impossible to retrofit such foil sheets.
Metallic foil has been used as thermal insulation. Thermal insulating structures using metallic foil insulation have been constructed in a variety of forms. For example, panels comprising a pair of thin metal walls having insulating filler therebetween are known, as illustrated by Woodson U.S. Pat. No. 1,626,655, issued May 3, 1927, and Finck U.S. Pat. No. 1,967,611, issued July 24, 1934. Additionally, metallic materials have been used as insulating fillers, such as ribbons of aluminum foil as described in Kiesel U.S. Pat. No. 2,175,630, issued Oct. 10, 1939, and particles coated with metallic paint, as described in Wasserback U.S. Pat. No. 2,896,278, issued July 28, 1959. More recently, however, metallic insulation has fallen into disuse, mainly because materials such as aluminum foil lose their reflective properties when they become wet or damp. Additionally, fibrous insulating materials such as fiberglass have higher R-values per unit cost, and have thus generally replaced metalic materials as thermal insulating materials.